Biofuel appliance venting system

ABSTRACT

A venting system for a biomass burning appliance. The system includes at least one pipe element formed of a steel alloy comprising: a maximum of 0.025 weight percent of Carbon, a maximum of 1.0 weight percent of Manganese; a maximum of 0.030 weight percent of Sulfur; a maximum of 0.1.00 weight percent of Silicon; a maximum of 0.035 weight percent of Nitrogen; a range of 17.50-19.50 weight percent of Chromium; a maximum of 1.00 weight percent of Nickel; a range of 1.75-2.5 weight percent of Molybdenum; and a range of a combination of Titanium+Columbium having a minimum of percentage of 0.20+4×(C+N) and a maximum of 0.80 percent.

BACKGROUND

Fuel burning appliances require an exhaust system to conduct combustionproducts including noxious gasses and water vapor to the exterior of adwelling.

Vent pipes, fittings and adapters, all exhaust systems generally includeone or more usually made from a ductile material, such as sheet metal.These components are assembled in place and installed to custom fit theexhaust system to a given space. Vent pipes are usually located betweenwalls, in attics and in crawl spaces where there is little room to work.As a result, the manipulation of the vent pipes is difficult,particularly with regard to connecting vent pipe sections.

One desirable type of combustion product for such appliances is biomass.Biomass fuels are organic and include seed, wood, crops, manure andgarbage. Typical biomass fuels which can be burned include corn andwood. Some biomass fuels emit an exhaust when burned that containscorrosive chemicals.

SUMMARY OF THE TECHNOLOGY

The present technology, roughly described, pertains to a venting systemfor a biomass burning appliance. The system includes at least one pipeelement formed of a steel alloy comprising: a maximum of 0.025 weightpercent of Carbon, a maximum of 1.0 weight percent of Manganese; amaximum of 0.030 weight percent of Sulfur; a maximum of 1.00 weightpercent of Silicon; a maximum of 0.035 weight percent of Nitrogen; arange of 17.50-19.50 weight percent of Chromium; a maximum of 1.00weight percent of Nickel; a range of 1.75-2.5 weight percent ofMolybdenum; and a range of a combination of Titanium+Columbium having aminimum of percentage of 0.20+4×(C+N) and a maximum of 0.80 percent.

The advantages of the present technology will appear more clearly fromthe following description in which the preferred embodiment of thetechnology has been set forth in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an exemplary implementation of a ventingsystem using the present technology.

DETAILED DESCRIPTION

A biofuel venting system is disclosed.

FIG. 1 shows a perspective view of an exhaust system incorporating thetechnology. FIG. 1 shows an exemplary appliance 100, which may be abiomass fuel burning appliance which is connected to an exhaust system.The exhaust system comprises pipe sections 200, 210, a storm collar 150and vertical termination 160. Pipe section 200 may be coupled to theappliance 100 by an appliance adapter 110. Sections 200 and 210 extendthe exhaust system through a section of the roof of a building 120, andpossibly through other exemplary structural elements including a ceilingfire stop 130, and flashing 140. Interlocking systems may be utilized tocouple the pipe sections 200, 210 to each other, to the applianceadapter 110 or the appliance 100 itself, to the vertical termination160, and the like. Note that the pipe sections shown in FIG. 1 may be ofthe type commonly referred to as single wall or double wall vent pipefor biomass fuel burning appliances.

In accordance with the technology, each pipe section component is formedof stainless steel having a composition as follows:

TABLE 1 Element Weight Percentage Carbon 0.025 max. Manganese  1.00 max.Phosphorus 0.040 max. Sulfur 0.030 max. Silicon  1.00 max. Nitrogen0.035 max. Chromium 17.50-19.50 Nickel  1.00 max. Molybdenum 1.75-2.50Titanium + Columbium 0.20 + 4 × (C + N) min.-0.80 max.

The aforementioned composition is commercially available under theUnified Numbering System (UNS) standard designation of type “S44400”stainless steel, commonly referred to as “444” stainless steel.

A number of stainless steel materials which have corrosive resistantproperties are known. Extensive testing under the supervision of theinventors has determined that the aforementioned material exhibitssuperior properties for biofuel burning appliance venting systems giventhe corrosive effects of these exhaust and the heat requirements of suchappliances.

Biofuel exhaust can cause both chemical corrosion and heat damage. Thechemical composition, and in particular, the corrosive elements in atypical biofuel exhaust, were determined before selecting and a numberof exhaust system material candidates were analyzed.

To develop the system, testing was first performed to determine thecorrosive components of combusting biofuel and in particular corn fuelsknown commonly as “feed corn”, i“stove corn” and “seed corn”. Combustionof such materials was compared to wood pellet fuels used as a controlFeed corn is comprised of whole yellow corn commercially available underthe trade name Grainland Select Whole Corn from Country Acres FeedCompany, Brentwood Mo. Stove corn is comprised of whole yellow corncommercially available under the trade name Valley View Feeds Stove cornfrom Balley View/Brubaker Grain, Farmersville Ohio. Seed corn is wholecorn that may be treated with one or more of Captan, Metalaxyl,Pirimiphos-methyl, Imidacloprid,1-[(6-chloro-3-pyridinyl)methyl]-N-Nitro-2-imidazolidinimine, andcommercially available under the tradename Gaucho® from Gustafson LLC,Plano Tex.; or treated with Fludioxonil, 4-(2,2-difluoro-q,3benzodioxol-4-yl)-1H-pyrrole-3-carbonitrile(R)-2[(2,6dimethylphenyl)-methoxyacetylamino]-propionic acid, methyl ester(mefenoxam) and Chlorpyrifos (Lorbsan), commercially available under thetrade name MaximXL® from Sygenta Crop Production Inc, Greensboro N.C.

Elevated levels of potassium chloride and potassium sulfate salts in theair emissions from corn combustion along with elevated levels of acidgases (sulfur dioxide and nitrous oxides) producing acidic emissionswere found. Such emissions were responsible for the corrosiveenvironment within the exhaust pipes burning corn biofuels. Tests ofcorroded pipe confirmed the corroded material on the pipe surfaces asbeing enriched in sulfur, chlorine and potassium. Analyses of bulk cornfuel confirm high levels of nitrogen, sulfur, chlorine, and potassium ascompared to typical wood pellets. This is demonstrated in Table 1 whichshows the results of air emission testing on three different fuels.

TABLE 1 Wood Analysis Units Pellets Feed Corn Stove Corn Nitrogen Oxidesg/hr 1.28 15.65 16.36 (NO_(x)) Sulfur Dioxide g/hr 0.02 2.89 1.77 (SO₂)Carbon Monoxide g/hr 32.05 7.91 11.05 (CO) NO_(x) g/kg 1.59 14.88 15.96SO₂ g/kg 0.03 2.75 1.72 CO g/kg 39.80 7.52 10.78 Particles* g/kg 1.841.10 2.35 Acid Equivalence equivalent 1.13E−02 4.18E−02 4.42E−02 s/kg pH(emissions)** pH units 3.31 2.30 2.25 Burn Rate kg/hr 0.805 1.051 1.025Avg. ° C. 144.1 115.0 103.4 Temperature*** Total Test Time hrs

Table 2 shows an analysis of the components making up the various typesof fuels:

TABLE 2 Analysis Method Units Wood Pellets Feed Corn Stove Corn SeedCorn pH (material)* 9045C pH 4.14 6.23 6.28 6.12 units Water Soluble365.3M ppm 47.2 154 127 179 Phosphate Water Soluble 300.0 ppm <25 10774.2 135 Sulfate Water Soluble 350.1M ppm 3.9 42 36 47 Ammonia WaterSoluble 300.0 ppm <50 <2.0 <2.0 <2.0 Nitrate + Nitrite Water Soluble300.0 ppm ~37 343 414 359 chloride Total Nitrogen 351.4M ppm 988 14,80011,200 12,400 Total Chlorine 9056 ppm <50 325 415 350 Ash (@ 550° C.) %0.15 1.17 1.27 1.16

An ion analysis of the emissions shown the following amounts of residualmaterials in a venting system:

TABLE 3 Feed Stove Analysis Units Wood Pellets Corn Corn Soluble mg/kgdry 6.44 13.28 15.87 Phosphate fuel Water Soluble Sulfate mg/kg dry70.90 120.90 153.60 fuel Water Soluble Ammonia mg/kg dry 0.36 22.7327.90 fuel Water Soluble mg/kg dry 2.44 <1.03 1.66 Nitrate + Nitritefuel Water Soluble Chloride mg/kg dry 10.51 130.20 276.49 fuel

Literature reports that normal dent corn, as it is a seed in contrast toa stalk, contains more than 9% protein. Protein is made up of aminoacids, which in turn have high levels of nitrogen. Further, agriculturalmaterials, like corn, have significantly more sulfur, chlorine,phosphorous, and potassium than wood. It is speculated that the highermoisture content of corn as compared to typical wood pellets may alsocontribute to the corrosion problem due to the increased potential ofcondensed water (with a low pH and corrosive salts in solution) being inprolonged and direct contact with pipe surfaces.

Also contributing to corrosive nature of corn combustion emissions arethe elevated phosphorous levels in corn producing phosphoric acid andphosphate salts upon combustion and the elevated levels of ammoniumsalts in the air emissions, most likely originating from the incompletecombustion of amino acids.

Conditions that increase the potential for condensation include cornwith unusually high moisture content, a long chimney allowing foremissions to cool, and a cold climate with a high relative humidity, ora combination of them. Similarly, corn with a high moisture content, ahigh salt content, a high heat content (creating more nitrogen oxide andperhaps some direct hydrochloric acid emissions), or the combustion ofexpired seed corn containing fertilizers or pesticides may be all or inpart responsible for the corrosion complaints.

Once the corrosive components of the corn were known, seven differentmetal coupon samples were tested in corrosive solutions designed tomimic the exhaust corrosion. These include: type 304 stainless steel;type 316L stainless steel; Allegany Ludlum material AL29-4C® (steel);type 444 stainless steel; type 436S steel; type 430 steel and copper.

Grade 304 is a versatile and most widely used stainless steel, availablein a wide range of products, forms and finishes. An exemplarycomposition of Grade 304 stainless steel includes:

TABLE 4 Element Weight Percentage Carbon 0.08 max. Manganese 2.00 max.Phosphorus 0.043 max.  Sulfur 0.030 max.  Silicon 0.75 max. Nitrogen0.10 max  Chromium 18.00-20.00 Nickel 8.00-11.00 max Molybdenum1.75-2.50

Grade 316 is a standard molybdenum-bearing grade stainless steel. Themolybdenum gives 316 better overall corrosion resistant properties thanGrade 304, particularly higher resistance to pitting and crevicecorrosion in chloride environments. Grade 316L is a low carbon versionof 316 and is resistant to sensitization (grain boundary carbideprecipitation). Thus it is extensively used in heavy gauge weldedcomponents (over about 6 mm). An exemplary composition of Grade 316L isas follows:

TABLE 5 Element Weight Percentage Carbon 0.03 max. Manganese 2.00 max.Phosphorus 0.045 max.  Sulfur 0.030 max.  Silicon 0.75 max  Nitrogen0.10 max  Chromium 16.00-18.00 Nickel 10.00-14.00 max Molybdenum  3.0max

The alloy designated AL 29-4C® is a stainless steel developed byAllegheny Ludlum. The alloy has excellent resistance to brackish,polluted or high chloride waters, e.g., seawater. AL 29-4C® alloy isknown to provide the following advantages over other competitivematerials: high resistance to severe chloride environments, such asseawater; higher resistance to vibration damage than titanium; higherresistance to erosion-corrosion than titanium and copper based alloys;better heat transfer properties than austenitic stainless steels; andlow cobalt content and cost effectiveness.

A typical composition of AL 29-4C® is given in Table 6.

TABLE 6 Element Weight Percentage Carbon 0.02 Manganese 0.5 Phosphorus0.03 Sulfur 0.01 Silicon 0.4 Nitrogen 0.035 max. Chromium 29 Nickel 1.00 max. Molybdenum 0.4 Cobalt 0.03 Titanium + Niobium 0.5

AISI Type 436 stainless steel is a ferritic stainless steel. The testsample was the Allegany Ludlum designation AL 436S® alloy is known tohave improved general corrosion and pitting resistance compared toferritic steels having less chromium. Like all ferritic stainless steelsthe AL 436S® alloy provides resistance to stress corrosion cracking inthe presence of chlorides.

A typical composition of AL 436S® is given in Table 7 below.

TABLE 7 Element Weight Percentage Carbon 0.01 Manganese 0.20 Phosphorous0.020 Sulfur 0.001 Silicon 0.37 Chromium 17.3 Nickel 0.30 Molybdenum1.20 Titanium 0.20 Nitrogen 0.015 Ti/(C + N) ≧ 8.0

Type 430 stainless steel combines good corrosion resistance with goodformability and ductility. It is a ferritic, non-hardenable plainChromium stainless steel with excellent finish quality. Grade 430 alsohas excellent resistance to nitric attack, which makes it well suited touse in chemical applications. An exemplary composition of Grade 430 isshown in Table 8:

TABLE 8 Element Weight Percentage Carbon 0.12 max Manganese 1.00Phosphorous 0.04 Silicon 1.00 Chromium 16.00-18.00 Nickel 0.75 maxMolybdenum 1.20

To determine the best material suitable for use in biomass fuelappliance, and in particular a corn burning appliance, a testing programwas implemented involving subjecting the metal samples to corrosiveaqueous solutions at varying concentrations and temperatures. Theaqueous solution (solution 1) was designed to mimic the liquidscondensing on a chimney, attached to an appliance combusting corn. Theprincipal agents of the solution being: nitric and sulfuric acid alongwith ammonium and potassium, inorganic salts. To accelerate thecorrosion testing, a concentrated version of the solution (solution 2)was used, which subjected the metal coupons to a more harsh environmentthan would be found in a chimney as a consequence of corn combustion.The concentrated solution was five times as concentrated as the firstsolution. Table 9 lists the composition of the solutions:

TABLE 9 Compound Formula Solution 1 Solution 2 Ammonium NH₄Cl 1.3 g 6.5g chloride Potassium K₂SO₄ 4.5 g 22.5 g sulfate Potassium KCl 7.4 g 37 gchloride Potassium KHCO₃ 5.8 g 29 g bicarbonate Nitric Acid HNO₃ 37.4 ml187 ml (70%) Sulfuric Acid H₂SO₄ 3.6 ml 18 ml (95-98%) Distilled waterH₂0 Upto Upto volume - 50 volume - 50 milliliters milliliters

The test solution was derived from an analysis conducted on corncombustion emissions. The highly corrosive environments created in theconcentrated solutions would likely cover any bio-fuel combustion:switch grass, grape seeds, cherry pits, bark, or other agriculturalbyproducts. The test setup consisted of placing sheet metal coupons inbeaker/watch glass cells with 50 milliliters of test of solution. Morethan 60 coupon samples subjected to three temperatures (roomtemperature, 200° F. (93° C.) and 570° F. (299° C.)) were used in thecorrosion testing. The elevated temperature sets cycled from roomtemperature to testing temperature each day of testing. In thehigh-temperature set, salt deposits were rinsed from the coupons andbeakers before adding new solution. And the mid-temperature set waschanged as needed, due to less evaporation. The room temperature sampleswere not changed, as little was lost from evaporation. It was apparentfrom the testing that some metals showed extreme signs of corrosion andothers showed little or no visible signs of corrosion. The corrosionsigns observed included: dissolving metal, pitting, and/or salts formingon metal surfaces. In addition, many of the solutions changed color dueto the leaching of metal components into solution.

After 110 hours of testing at 200° F., corrosion is evident on all butthe AL29-4C® and 444 samples. After 41 hours of corn corrosion screeningat 313° F. (156° C.), then increased to 570° F. (299° C.) for 20 hours,some corrosion is evident on all samples. After 1344 hours (56 days) ofcorn corrosion screening at room temperature—between 70° F. (21° C.) and100° F. (38° C.), mild corrosion is evident on all samples except the444 and AL29-4C® samples.

Although the 444 and AL29-4C® samples performed equally well, it wasdetermined that the 444 material would make a superior venting systemfor a biomass fuel having the exhaust characteristics including thecompounds in relative percentage as that of the test solutions. The 444material provides a superior resistance to heat and is more costeffective. The UL standard for venting used on corn-burning appliancesrequires that the vent be tested to a continuous temperature of 570° F.with brief (10 minutes) forced-fire to 1700 F. The Allegheny Ludlumspecification sheets for AL29-4C note that the maximum use temperatureshould be restricted to 600 F. Also, the Allegheny Ludlum spec sheet for444 material notes that 444 may experience embrittlement if subjected tolong-term exposures of 885° F., but was able to handle “many years” ofexposure at 650° F. without degradation. Additionally, any embrittlementcould be reversed with short exposures of around 1200 F.

The foregoing detailed description of the technology has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the technology to the precise form disclosed.Many modifications and variations are possible in light of the aboveteaching. The described embodiments were chosen in order to best explainthe principles of the technology and its practical application tothereby enable others skilled in the art to best utilize the technologyin various embodiments and with various modifications as are suited tothe particular use contemplated. It is intended that the scope of thetechnology be defined by the claims appended hereto.

1. A venting system for a biomass burning appliance, comprising: atleast one pipe element formed of a steel alloy comprising: a maximum of0.025 weight percent of Carbon, a maximum of 1.0 weight percent ofManganese; a maximum of 0.030 weight percent of Sulfur; a maximum of1.00 weight percent of Silicon; a maximum of 0.035 weight percent ofNitrogen; a range of 17.50-19.50 weight percent of Chromium; a maximumof 1.00 weight percent of Nickel; a range of 1.75-2.5 weight percent ofMolybdenum; and a range of a combination of Titanium+Columbium having aminimum of percentage of 0.20+4 ×(C+N) and a maximum of 0.80 percent. 2.The system of claim 1 further including a biomass burning applianceburning a biomass fuel.
 3. The system of claim 1 wherein the at leastone pipe element comprises an exhaust system including at least a onecylindrical pipe sections pipe sections, a storm collar and a verticaltermination, each of which is comprised of the steel alloy.
 4. Thesystem of claim 3 wherein the at least one pipe element further includesan appliance adapter 110 comprised of the steel alloy.
 5. The system ofclaim 4 wherein the biomass fuel produces an exhaust containing at leastthe following NH₄C₁, K₂SO₄, KCl, KHCO₃, HNO₃ and H₂SO₄.
 6. The system ofclaim 4 wherein the biomass fuel produces an exhaust containing at leastone of NH₄Cl, and KCl, and at least one of HNO₃ and H₂SO₄.
 7. A biomassfuel burning heating system, comprising: a biomass fuel applianceproducing an exhaust containing at least one of NH₄Cl, and KCl, and atleast one of HNO₃ and H₂SO₄; and an exhaust system including at leastone cylindrical pipe sections, a storm collar and a verticaltermination; each of said sections, collar and termination beingcomprised of steel including at least a maximum of 0.025 weight percentof Carbon, a maximum of 1.0 weight percent of Manganese; a maximum of0.030 weight percent of Sulfur; a maximum of 1.00 weight percent ofSilicon; a maximum of 0.035 weight percent of Nitrogen; a range of17.50-19.50 weight percent of Chromium; a maximum of 1.00 weight percentof Nickel; a range of 1.75-2.5 weight percent of Molybdenum; and a rangeof a combination of Titanium+Columbium having a minimum of percentage of0.20+4×(C+N) and a maximum of 0.80 percent.
 8. The system of claim 7wherein the appliance includes a burning biomass fuel consistingprimarily corn.
 9. A biomass fuel burning heating system, comprising: abiomass fuel appliance producing an exhaust containing at least one ofNH₄Cl, and KCl, and at least one of HNO₃ and H₂SO₄; and an exhaustsystem including at least one cylindrical pipe sections, being comprisedof steel including at least a maximum of 0.025 weight percent of Carbon,a maximum of 1.0 weight percent of Manganese; a maximum of 0.030 weightpercent of Sulfur; a maximum of 1.00 weight percent of Silicon; amaximum of 0.035 weight percent of Nitrogen; a range of 17.50-19.50weight percent of Chromium; a maximum of 1.00 weight percent of Nickel;a range of 1.75-2.5 weight percent of Molybdenum; and a range of acombination of Titanium+Columbium having a minimum of percentage of0.20+4 ×(C+N) and a maximum of 0.80 percent.
 10. The system of claim 9wherein the biomass fuel produces an exhaust containing at least thefollowing NH₄C₁, K₂SO₄, KCl, KHCO₃, HNO₃ and H₂SO₄.
 11. The system ofclaim 10 wherein the biomass fuel produces an exhaust with a temperaturein the range of about 0-650 degrees Fahrenheit.